Flash memory devices may store information in the form of electrical charge. Each cell of the flash memory devices may have a substrate, a transistor gate and an additional floating metal transistor gate between the substrate and the transistor gate. Charge may be stored in the floating gate during programming by injecting electrons into the floating gate. Cells are typically programmed in groups, for example, written page-by-page and erased block-by-block. In single-level cell (SLC) devices, each cell may either be programmed with charge or may remain un-programmed (erased), thus effectively defining two binary states to store one bit of information per cell. In multi-level cell (MLC) devices, each cell may store more than one bit of information at each transistor. As the charge in the floating gate may vary continuously, MLC devices may apply electrical charge to the floating gates in one of multiple (n) levels. Thus, an (n)-level flash cell, where n=2^k, may store k bits of information, where each combination of k bits is mapped to one of n program levels (e.g. via Gray coding).
The amount of electrical charge stored in the floating gate cell may be directly related to the voltage level used to fully conduct current across the cell's transistor gate. Therefore, the amount of charge in a cell may be measured by the minimal gate voltage level used for conduction (e.g., also known as threshold voltage, VT). Since the actual charge level may vary among cells programmed to a certain program level, the specific threshold voltage may also vary among these cells.
Flash memory devices may be read by comparing the threshold voltage of cells to one or more read thresholds or decision levels. The read thresholds may distinguish a plurality of different voltage ranges, each associated with a different one of (n) program levels of the multi-level cell. Read operations may determine within which voltage range is the threshold voltage for conducting each cell, and therefore, the associated program level and value of the cell.
During the life span of a flash memory device, the insulating properties of the floating gates may degrade and electrons may escape from the cells causing the cell charge to shift or decrease, which may be referred to as “charge loss.” Charge loss or other errors in the charge stored in cells may result from programming errors (e.g., either intentional or not), age, storage temperature, repeated use of program/erase cycles, retention and various other reasons. Charge loss may cause a shift in the charge of cells, which may cause errors when read by un-shifted thresholds previously generated for fully charged cells.